Vibrating machine

ABSTRACT

A machine for finishing parts by vibrating the parts in the presence of an abrasive media. The machine comprises a rounded bottom tube which is pivotably suspended in a frame around the tub, the suspension being by means of axially aligned shafts which project from end walls of the tub. These shafts are offset to one side of the tub center of gravity. Along the bottom of the tub on the opposite side of the center of gravity are three eccentric weights rotated on axes which are parallel with each other and with the tub pivot axis. The weights are so phased as to produce centrifugal forces which effectively vibrate the tub, and at the same time minimize the vibration transmitted to the frame around the tub.

United States Patent 3,183,630 5/1965 Wright 3,253,369 5/1966 Reichertetal.

ABSTRACT: A machine for finishing parts by vibrating the parts in the presence of an abrasive media. The machine comprises a rounded bottom tube which is pivotably suspended in a frame around the tub, the suspension being by means of axially aligned shafts which project from end walls of the tub. These shafts are offset to one side of the tub center of gravity. Along the bottom of the tub on the opposite side of the center of gravity are three eccentric weights rotated on axes which are parallel with each other and with the tub pivot axis. The

weights are so phased as to produce centrifugal forces which effectively vibrate the tub, and at the same time minimize the vibration transmitted to the frame around the tub.

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- sum 1 BF 5 INVENTOR. CHARLES E. JONES ATTORNEYS PATENTEDHAY25I9W 9579.919

' SHEET 2 OF 5 FIG. 2

' INVENTORQ I CHARLES E. JONES ATTORNEYS.

PATENTEDHAY25197| SHEET 3 BF 5 INVENTOR. CHARLES E. JONES 7"% FIG.3

ATTORNEYS PATENTED HAYZS I9?! sum 0F 5 INVENTOR. CHARLES E. JONES ATTORNEYS RATING MACHINE The present invention relates to vibrating machines, more particularly to vibratory finishing machines.

The invention is particularly applicable to finishing machines of the type in which a part to be finished is vibrated in the presence of a finishing media, and will be described with particular reference thereto, although it will be appreciated that the invention has broader applications.

Vibratory finishing is well known, and usually is carried out in an open-top tub which is vibrated so that it moves in an orbital path. With his type of movement, the inside surface of the tub frictionally engages the mass within the tub (the finishing media and parts to be finished) predominantly in one direction. This forces the mass to flow upwardly along a wall of the tub, until at a certain elevation it becomes unsupported and falls over or cascades onto itself, the relative movement of media and parts providing the finishing action. The system is applicable to other processes than finishing such as polishing, deburring bumishing, flash removal, descaling and cleaning.

Many devices have been employed to vibrate the tubs, one such device or mechanism consisting of an eccentric rotating shaft supporting the tub so that as the shaft rotates the tub moves in an orbital path. The problem with theuse of this type of mechanism is that very heavy counterweights had to be mounted on the shafts to counterbalance the combined weight of the tub and mass in the tub. This reduced the forces exerted against bearings supporting the shafts, but even then large nd costly bearings were required, and bearing life was short. In addition, the counterweights placed an upper limit on the size or capacity to which the machine could be built.

A further problem experienced was that the tub and counterweight arrangement for the machine had to be supported in a subframe which in turn was cushion mounted in an external frame to prevent excessive vibration in the floor mounting for the machine. The need of a subframe, which need is not limited only to the eccentric shaft-type of machine, adds to the cost and complexity of the machine.

As another disadvantage experienced with most prior machines, although the frequency of vibration in the machine could be changed by varying the speed of rotation of the drive mechanism, there was no practical way to change the amplitude of vibration.

These and other disadvantages are overcome in accordance with the present invention by providing in a machine of the general character described including a vibrating tub adapted to be supported in a floor-mounted frame; the improvement comprising a pivot axis for the tub spaced from the tub center of gravity; and multiple force-imparting means mounted on the tub spaced from said pivot axis; said force-imparting means being so phased as to exert in succession strong moment forces about said pivot axis alternately in opposite directions, and simultaneous with said moment forces a smaller force either towards or away from the pivot axis.

Preferably the force-imparting means comprises three eccentric weights rotatable at the same speed about parallel axes parallel to the tub pivot axis, two of the eccentric weights being rotatable in opposite directions and phased to exert a centrifugal force in the same direction when the direction is approximately at right angles to a plane between the pivot axis and the force-imparting means; and in opposite directions when those directions are parallel with said plane; the third eccentric weight exerting a centrifugal force towards or away from the pivot axis when the first two eccentrics exert centrifugal forces in the same direction.

Preferably the pivot axis for the tub is spaced diagonally to one side of and above the tub center of gravity, the eccentrics comprising cylindrical parallel rods having their axis of rotation removed from the axis of the rods, the eccentrics being positioned near the bottom of the tub on the side of the center of gravity opposite from the tub pivot axis.

In accordance with an aspect of the invention, the vibrating machine comprises a motor mounted on the frame thereof, an intermediate driving means rotatably mounted coaxial with tub pivot axis, and means connecting said driving means to said motor and to the force-imparting means to rotate the latter.

It will become apparent that the vibrating mechanism in accordance with the present invention effects an orbital movement of the tub which is highly efficient for finishing and other processes, and which at the same time results in little vibration at the tub pivot axis, so that the tub can be mounted directly in a floor frame for the machine. Eliminated is the need for a subframe, spring mountings, and counterweights, permitting the construction of a lightweight machine which can be moved easily from one place to another in a shop, for either batch or production line use. Alternatively, the invention permits the construction of a machine of very large size.

Accordingly, it is an object of the present invention to provide a vibrating machine in which the disadvantages of prior machines are overcome.

In particular, it is an object of the present invention to provide a vibrating machine which is light in weight, simple, and easily movable from place to place in a shop, and in which the need for counterweights and/or subframe is avoided.

A further object of the invention is to provide a vibrating machine the design of which can be adapted for various applications, for instance for small or very large sized pieces.

It is also an object of the invention to provide a vibrating machine in which the amplitude as well as the frequency of vibration of the machine can be varied.

The invention, objects and other advantages thereof will become apparent upon further consideration of the following specification, with reference to the accompanying drawings, in which FIG. 1 is a section side elevation view of a vibrating machine in accordance with the present invention;

FIG. 2 is a section front view taken along line 2-2 of FIG. 1;

FIG. 3 is a side elevation view opposite the view of FIG. 1 of the vibrating machine in accordance with the present invention;

FIG. 4 is a section view taken along line 4-4 of FIG. 2;

FIG. 5 is a section view taken along line 5-5 of FIG. 2;

FIG. 6 is a section view taken along line 6-6 of FIG. 4; and

FIG. 7 is a detailedsection view illustrating principles in accordance with the present invention.

Turning to the drawings wherein the showings are for the purpose of illustrating the preferred embodiment of the invention only and not for the purpose of limiting the same, the FIGS. show a vibrating machine A which has a floor-supported frame B, and a vibrating tub C, in which parts to be finished and a finishing media are placed, mounted in the frame. Also part of the vibrating machine is a drive motor mechanism D by which the tub is vibrated in an orbital path in a way to flow the finishing media and parts in a cascading manner within the tub.

The frame B of the tub is rectangular in shape in both elevation and plan views (note FIGS. 1 and 2) and comprises four vertical or upright L-shaped channels 10 defining the four corners of the machine, and a plurality of horizontal front, rear and side cross braces 12, also in the form of L-shaped channels. The braces 12 are located at the top of the machine, near the bottom, and about halfway up on the sides.

The machine may be enclosed with a plurality of panels for decorativeand safety purposes, but should be open at the top, for loading. Within the frame, encompassed by the four sides, is the elongated U-shaped tub C (shown clearly in FIG. 4) having a front wall 16, a rear wall 18, a rounded bottom 20, and end walls 22 and 24 (FIG. 2) between which the front and rear walls and bottom extend. Aside from the Ushape for the tube, no special configuration is required.

Welded onto the end walls 22 and 24, somewhat diagonally forward and above the tub center of gravity (with reference to the section or end views of FIGS. 1, 2 and 3) the tub C is provided with a pair of axially aligned oppositely projecting supporting shafts 26 and 28 welded to the end walls defining the tub pivot axis. The lengths of the supporting shafts are sufficient to bring them out to or beyond a pair of horizontal end cross braces 30 and 32, each of these braces supporting a bearing housing 34, 36 into which the shafts extend.

FIG. illustrates the right-hand support arrangement (when viewed from the front of the machine) in detail, the bearing housing 36 having an inner bearing sleeve 38 within which the shaft 28 is rotatably mounted. The shaft is integral with a plate 40 which is of greater diameter than the shaft, and which is welded to the end wall 24 of the tub.

Still referring to FIG. 5, the shaft 28 has an inner relatively large diameter portion 42 next to the tub and a lesser diameter portion 44 spaced from the tub, which is the portion encompassed by the bearing housing 36. On the larger diameter portion 42, a cylindrical sleeve 46 is rotatably mounted supported by a pair of spaced cylindrical bearings 48. At one end of the sleeve 46, furthermost removed from the tub end wall, is a pulley 50 welded to the sleeve, and at the innermost end, adjacent the tub end wall, is a gear 52 also welded to the sleeve. By means of the bearings 48, the sleeve, pulley and gear are freely rotatable together around the shaft extension.

Keyed to the smaller diameter portion, on the very end of the shaft 28 and outboard of the bearing housing 36 is a second gear 53, to be described.

At the opposite left-hand end of the tub, the supporting arrangement consists only of the shaft 26 similar in part to shaft 28 and the bearing housing 34 which is similar to housing 36.

Below the tub mounted on a motor bracket 54 (FIGS. 1 and 3) is the drive motor mechanism D including a belt pulley 56. By means of a continuous belt 58, the mechanism drives the pulley 50 (FIG. 5 rotatable about the supporting shaft 28.

It was mentioned that the tub pivot axis is spaced from the tub center of gravity somewhat diagonally towards the upper front side of the tub. The vibrating mechanism for the tub is positioned almost diametrically opposite the pivot axis, along the bottom rear side of the tub, and comprises three rotating eccentrics 60, 62 and 64, shown in FIGS. 3 and 4, details of the eccentrics being shown in FIG. 6. Each eccentric is in the form of an elongated cylindrical member 66 provided with oppositely extending aligned shafts 68 and 70 offset from the axis of the cylindrical member but parallel therewith. The eccentrics are housed within cylindrical receptacles 72 having at opposite ends thereof bearing supports 74 rotatably engaging the shafts 68 and 70. In this way, each eccentric is comprised of a large longitudinally extending weighted side offset from its axis of rotation. The receptacles 72 in turn are supported between end brakes 76 welded to the end walls of the tub C adjacent the rounded bottom of the tub. Both ends of the receptacles are covered with plates 78, the right end plate (in FIG. 6) being provided with an aperture 80 through which the shaft 70 projects. Affixed to the outer exposed end of the shaft 70 is a gear 82, the spacing of the eccentrics and diameters of the gears being such that the latter intermesh as shown in FIG.

Referring to this FIG. and also FIG. 4, it is apparent that the first and last eccentrics 60 and 64 will rotate in one direction, clockwise or counterclockwise and the intermediate eccentric 62 will rotate in the opposite direction.

Between the first of the eccentrics (item 60), in particular the gear 82 thereof, and the gear 52 (FIG. 5) mounted on the tub shaft 28, is a gear train (FIG. 1) including two driven gears 84 and 86 mounted on short rods welded to the end wall of the vibrating tub. Through the belt 58, the motor mechanism D causes the pulley 50 and gear 52 to turn, in turn rotating the gears 84 and 86, and the three eccentrics through meshed gears 82.

Also shown in FIG. I is a horizontal bracket 88 near the bottom of the machine attached to one of the frame uprights, this bracket providing a pivot axis for a downwardly extending motor mounting 90 to which the motor bracket 54 is connected. An upright arm 92 is fixed integral with the motor mounting, this arm in turn being connected with a speed-adjusting nut 94 threaded onto a rotatable horizontal rod 96. The rod extend between the front and rear sides of the machine and is rotatably mounted in bearings 98 at both sides.

At the front end of the rod a handwheel 102 is provided. By turning the handwheel, the rod 96 is turned, in turn advancing the speed-adjusting nut 94 either to the right or left, pivoting the arm 92 on the bracket 88. So pivoting the arm causes the motor mounting to pivot also, towards or away from the gear 52, increasing or decreasing the tension in the belt 58. This in turn increases or decreases the speed of the eccentrics by greater or less slippage in the belt.

Attached to the speed-adjusting nut 94 is a pointer 104 positioned in front of a speed-indicating means 106 to indicate to the operator the speed at which the tub is being vibrated.

With reference to FIGS. 5, 1 and 2, the tub-supporting shaft 28 which supports the drive gear particular and pulley 50, also supports at its terminal end, a gear 53 mentioned above. This gear is in the form of a worm wheel, and engages a worm gear 108 mounted on a dumping rod 110 which extends between the front and rear sides of the machine supported in bearings 112. The worm gear is adapted to turn with the rod, but is slidable along the rod and positioned on the rod lengthwise by a pair of coil compression springs 114 on opposite sides of the worm gear, one coil spring being seated between the worm gear and the front side of the machine, the other spring being seated between the worm gear and a sleeve 116 also encompassing the rod. The latter is positioned against an adjusting nut 118 threaded onto the rod, and by turning the adjusting nut, the compression in the springs can be varied. Also mounted on the rod is a handwheel 120 on the front side of the machine.

Rotation of the handwheel causes the worm 108 to rotate worm wheel 53, in turn causing the tub to pivot about its pivot axis. This permits separation of media nd parts, in a manner to be described. By spring holding the worm 108 on the dumping rod 110, the worm wheel 53 can be allowed to oscillate with vibration of the tub, without this vibration or oscillation being transmitted to the dumping rod 110 and machine frame.

Shown in FIG. 4 is a spring-loaded dashpot 122 mounted against a channel 124 in the front of vibrating machine. The piston member 126 of the dashpot abuts a pin 128 fastened to the bottom front side of the vibrating tub between bracket plates 130 (note FIG. 2) projecting from the tub front side. It will be recalled that the tub pivot axis is offset from its center of gravity. The purpose of the dashpot is to provide a resilient support against which the tub is nonnally biased by gravity.

Also resisting the normal biasing of the tub is a vertically extending rod 132 hinged to the tub at a point 133 diagonally above and to the front of the two tub pivot axes 26, 28 (notice FIG. 3). The rod is pulled downwardly by a spring 134 which is under compression bearing against the underside of a bracket 136 mounted on the machine frame. In this way the rod exerts a clockwise force on the tub (with reference to FIG. 3) which resists the counterclockwise force of gravity. The compression of the spring 134 is adjustable by turning the adjusting nut 138.

In operation, the tub C is first filled with media and the parts to be finished, up to a predetermined level in the tub. The motor D is started, causing the eccentrics 60-64 to rotate within their housings. In accordance with the invention, this rotation is in phase such as to exert substantial moment forces successively in opposite directions about the tub pivot axis and simultaneous with the moment forces smaller forces either towards or away from the tub pivot axis. FIG. 7 illustrates a first setting of the eccentrics, the weighted sides of the first and second eccentrics 60 and 62 both being in about the 2 0- clock position rearwardly directed and roughly at right angles to a plane between the pivot axis for the tub and the eccentrics. The third eccentric 64 has its weighted side oriented towards the l 1 oclock position towards the pivot axis of the vibrating tub.

As the eccentrics are rotated, the first and third eccentrics 60, 64 in a counterclockwise direction and the second eccentric 62 or intermediate one ire ti clockwise direction, it is apparent that at 90 with respect to the FIG. 7 positions the weighted sides of the first and second eccentrics will face in opposite directions, towards and away from the pivot axis for the vibrating tub respectively,and the weighted side of the third eccentric will be forwardly directed roughly at right angles to the plane between the eccentrics and the pivot axis.

As the eccentrics reach positions 180 with respect to those of FIG. 7, again the weighted sides of the first two eccentrics are oriented in the same direction, this time forwardly at about right angles to the plane between the eccentrics and the pivot axis, and the third eccentric will be oriented with its weighted side at the 5 oclock position parallel with the plane.

Finally at about the 270 positions, the weighted sides of the first and second eccentrics are again in opposite directions away from and towards the pivot axis while the third eccentric has its weighted side in about the 2 o'clock position.

It is apparent that the two eccentrics 60 and 64 produce successive substantial moments of forces about the pivot axis of the tub which throw the tub bottom rearwardly and forwardly, but produce no significant resultant force towards or away from the tub pivot axis. The third eccentric 64 complements the forces of the first two eccentrics, exerting a force towards the pivot axis of the tub when the tub is being pivoted rearwardly, pressing the bottom of the tub into engagement with the media and parts being finished, and away from the pivot axis when the tub is pivoted forwardly, pulling he bottom of the tub out of engagement with the media and parts. By suitably shaping the eccentrics the tub bearing supports are subjected only to that force necessary to obtain directional flowing of media and parts'within the tub, while at the same time, any desired moment of force for forward or rearward pivotal movement of the tub can be obtained.

In operation, with violent agitation of the tub, the vibration at the tub pivot and in the lightweight frame is so slight that the frame sits unfastened on the floor with no tendency to walk.

This is accomplishedwithout the necessity of a subframe, counterweights, or complex spring supports.

During operation of the machine, dashpot 122 simply provides a resilient member against which the tub is pivoted, and does not restrain the movement of the tub. Also, the springs 114 which hold the worm gear axially along the dumping shaft 110 are sufficiently weak to provide little or no dampening of the tub vibration, permitting the worm wheel 53 to oscillate with forward and rearward pivotal movement of the tub.

ln FIG. 4, there is illustrated a screen 140 extending downwardly and rearwardly into the vibrating tub supported on a frame 142 which is held against the upper edge of the front wall of the tub and between the tub end walls, the screen extending the full length of the tub. The purpose of the screen is to provide a means for separating media and parts being finished. Normally the. level of parts and media is inclined along line a as shown so that there is no flow through the screen. By pivoting the tub rearwardly with handwheel 120 (rotating dumping rod 110 and worm 108), the level of the media in the tub is changed to the line b, causing the media and parts to flow into the screen. With a suitable mesh, the parts can simply be picked ofi the screen, the media passing on through it.

Although the vibrating machine shown is particularly suitable for batch or job shop use, it should be apparent that the machine can be adapted readily for production line use and can be built in much larger sizes. The reason for this is that there are no weight restrictions such as those which would occur if counterweights were required, limiting the size of the machine. In particular, a very long machine can be built using the principles of the invention to deburr long rods and pipe. This is not possible with conventional counterweighted machines, and because of the simplicity of the machine of the present invention, it can be custom built as easily to suit many other needs.

If desired, the machine can be completely pivotable for dumping, or can be provided with a door in an end wall for dumping on a batch basis.

It was mentioned that the frequency of vibration can be varied simply by tightening the belt 58. The amplitude of vibration can also be varied by simply changing the offset weight of the eccentrics.

Also although operation of the machine of the present invention has been described with reference to particular settings and a particular phase relationship of the eccentrics, other settings or different phase relationships may be used to obtain specific results. The significant feature of the present invention is that three eccentrics are used to minimize forces on the tub pivot axis and at the same time maximize tub vibration. In this respect a fourth eccentric, or more, could be used to complement the forces achieved with the three eccentrics and obtain additional specific results.

Although the invention has been described with respect to specific embodiments, variations within the scope of the-following claims will be apparent to those skilled in the art.

Iclaim:

1. A vibrating machine comprising a tub,

frame means supporting the tub;

a pivot axis for the tub spaced from the tub center of gravimultiple force-imparting means mounted on the tub spaced from the pivot axis, said force-imparting means including first and second means to exert strong moments of force alternately in opposite direction in a plane substantially at right angles to the plane between the force imparting means and the pivot axis; and

a third means to exert smaller forces towards and away from the pivot axis when either one of said moments of force is being exerted.

2. The machine of claim 1 wherein said force-imparting means comprises three eccentrics rotatable at the same speed about parallel axes parallel to the tub pivot axis, two of the eccentrics being rotatable in opposite directions and in phase to exert a force in the same direction when that direction is approximately at right angles to a plane between the pivot axis and the force-imparting means; and in opposite directions when those directions are parallel with said plane; the third eccentric being with the first two eccentrics to exert a force towards or away from the pivot axis when the first two eccentrics exert forces in the same direction.

3. The machine of claim 2 wherein the pivot axis for tub is spaced diagonally above the tub center of gravity towards the upper rear corner of the tub.

4. The machine of claim 2 wherein said eccentrics comprise parallel cylindrical rods, and axially aligned shafts at the ends thereof about which the rods are rotated, the shafts being offset from the axis of the rods.

5. The machine of claim 4 wherein the cylindrical rods are mounted on the tub near the bottom of the tub opposite from the tub pivot axis.

6. The machine according to claim 2 comprising a motor mounted on the frame means thereof, driving means rotatably mounted coaxial with the tub pivot axis, and means connecting said driving means to said motor and to the eccentrics to rotate the latter.

7. The machine according to claim 6 including means to rotate the tub about the pivot axis thereof in addition to said force imparting means and during vibration of the tub.

8. The machine of claim 7 including a screen;

the tub normally being in an upright position, said means to rotate the tub being adapted to rotate the tub from the upright position, further including frame means holding said screen adapted to support the screen in a predetermined position within the tub by which media within the tub is caused to flow below the screen when the tub is in its normal upright position, and through the screen when the tub is rotated from its upright position.

9. The vibrating machine of claim 1 including a dashpot against which he tub is normally biased by the force of gravity.

10. A vibrating machine for finishing parts by vibrating the parts in the presence of an abrasive media comprising a rounded bottom tub in which the parts and abrasive media are placed;

said tub having opposed end walls, front and rear sides between the end walls and a rounded bottom;

axially aligned shaft means projecting form said tub end walls;

a frame encompassing said tub;

bearings on said frame into which said shaft means project;

said shaft means being offset above and to the front of the tub center of gravity;

three eccentrically shaped rods rotatable on axes parallel with the tub pivot axis mounted on the bottom of the b on the opposite side of the center of gravity from the pivot ternate forces in opposite directions when those directions are generally parallel with the plane;

the other end rod providing a driving force towards or away from the pivot axis when said one rod and center rod are weighted in the same direction. 

1. A vibrating machine comprising a tub, frame means supporting the tub; a pivot axis for the tub spaced from the tub center of gravity; multiple force-imparting means mounted on the tub spaced from the pivot axis, said force-imparting means including first and second means to exert strong moments of force alternately in opposite direction in a plane substantially at right angles to the plane between the force imparting means and the pivot axis; and a third means to exert smaller forces towards and away from the pivot axis when either one of said moments of force is being exerted.
 2. The machine of claim 1 wherein said force-imparting means comprises three eccentrics rotatable at the same speed about parallel axes parallel to the tub pivot axis, two of the eccentrics being rotatable in opposite directions and in phase to exert a force in the same direction when that direction is approximately at right angles to a plane between the pivot axis and the force-imparting means; and in opposite directions when those directions are parallel with said plane; the third eccentric being with the first two eccentrics to exert a force towards or away from the pivot axis when the first two eccentrics exert forces in the same direction.
 3. The machine of claim 2 wherein the pivot axis for tub is spaced diagonally above the tub center of gravity towards the upper rear corner of the tub.
 4. The machine of claim 2 wherein said eccentrics comprise parallel cylindrical rods, and axially aligned shafts at the ends thereof about which the rods are rotated, the shafts being offset from the axis of the rods.
 5. The machine of claim 4 wherein the cylindrical rods are mounted on the tub near the bottom of the tub opposite from the tub pivot axis.
 6. The machine according to claim 2 comprising a motor mounted on the frame means thereof, driving means rotatably mounted coaxial with the tub pivot axis, and means connecting said driving means to said motor and to the eccentrics to rotate the latter.
 7. The machine according to claim 6 including means to rotate the tub about the pivot axis thereof in addition to said force imparting means and during vibration of the tub.
 8. The machine of claim 7 including a screen; the tub normally being in an upright position, said means to rotate the tub being adapted to rotate the tub from the upright position, further including frame means holding said screen adapted to support the screen in a predetermined position within the tub by which media within the tub is caused to flow below the screen when the tub is in its normal upright position, and through the screen when the tub is rotated from its upright position.
 9. The vibrating machine of claim 1 including a dashpot against which he tub is normally biased by the force of gravity.
 10. A vibrating machine for finishing parts by vibrating the parts in the presence of an abrasive media comprising a rounded bottom tub in which the parts and abrasive media are placed; said tub having opposed end walls, front and rear sides between the end walls and a rounded bottom; axially aligned shaft means projecting form said tub end walls; a frame encompassing said tub; bearings on said frame into which said shaft means project; said shaft means being offset above and to the front of the tub center of gravity; three eccentrically shaped rods rotatable on axes parallel with the tub pivot axis mounted on the bottom of the b on the opposite side of the center of gravity from the pivot axis; means driving the rods at the same speed, the end rods in one direction and the center rod in the opposite direction; one of the end rods and the center rod being in phase so that the weighted sides thereof provide alternate forward and rearward moments of force in the same direction when that direction is approximately at right angles to the plane between the eccentrics and the tub pivot axis, and alternate forces in opposite directions when those directions are generally parallel with the plane; the other end rod providing a driving force towards or away from the pivot axis when said one rod and center rod are weighted in the same direction. 